1. Field of the Invention
The present invention relates generally to metal matrix compositions. Such compositions or composites comprise one or more base material metals such as, for example, aluminum, titanium, or magnesium, to which is added a selected percentage of ceramic material to alter the properties of the base material metal(s) in a positive manner. Strength, hardness, and drawability are increased. Drawability facilitates fabrication of various articles of manufacture from such composite materials. More specifically, the present invention pertains to an improved metal matrix composite which, in a preferred embodiment, uses boron carbide as the added ceramic material. The composites result from a novel method of manufacture producing a composite which is lighter, stronger, stiffer, and which has a higher fatigue strength than other available alloys of the base material metal, and which is also lighter, stronger, stiffer, and which has a higher fatigue strength than prior art metal matrices, composites, and particularly those metal matrix composites which are of comparable cost.
2. Prior Art
In recent years metal matrix compositions or composites have become popular materials for a variety of applications. This new family of materials has become popular because of improvements in stiffness, strength, and wear properties. Basic metal matrix composites are made typically with aluminum, titanium, or magnesium as the base material metal. Then certain percentages of ceramics are added. Typical ceramics include boron carbide, silicon carbide, titanium diboride, titanium carbide, aluminum oxide, and silicon nitride. Most known metal matrix composites are made by introducing the ceramics into the molten metal. In large production runs of metal matrix composites, the ceramic reinforcement must be wetted by the liquid metal to facilitate incorporation of the reinforcement into the melt. In those metal matrix composites using silicon carbide and aluminum, the silicon carbide is thermodynamically unstable in molten aluminum which leads to the formation of aluminum carbide at the interface and increased concentration of silicon in the material matrix during the solidification process. This interface reaction is believed to have detrimental effects on the mechanical properties of the resulting composite by reducing the interface strength and changing the composition.
Recently, powder metallurgy consolidation has emerged as a competing method of fabricating metal matrix composites by consolidating the powders by means of hot pressing and conventional powder metallurgy operations with vacuum sintering used to achieve a high density green body. By following certain isopressing and sintering techniques, a 99% theoretical density billet can be achieved.
In the present invention, it has been found that the most desirable ceramic candidate for metal matrix composites is boron carbide or silicon carbide. Boron carbide is the third hardest material known and the hardest material produced in tonnage. Boron carbide powders can be formed by a variety of reactions including the carbon reduction of any of several boron-oxygen compounds including boric oxide, borax, boracite, as well as by the direct combination of the elements. Usually, most commercial boron carbide is produced in arc furnaces. Boric acid is added together with carbon in the form of coke and heated to very high temperatures. An electric arc is maintained between graphite electrodes inside a furnace. The synthesis reaction is accompanied by the release of large volumes of carbon monoxide. Venting and disposal of the carbon monoxide gas constitutes a major design consideration. Boron carbide is also the lightest of the ceramics typically used in metal matrix composite technology, but it is very hard and expensive. Its hardness limits its extrudability. Thus it would be highly advantageous if it were possible to produce an improved metal matrix composite which utilizes an advanced ceramic such as boron carbide but which, unlike the prior art, results in an extrudable composite material that allows easy fabrication of various articles of manufacture so that such resulting articles have the specific strength and stiffness improvements as compared to equivalent articles of manufacture using only the base material metals.